1. Field of the Invention
The present invention relates to a power steering apparatus which applies a steering assist force to a steering mechanism by a hydraulic pressure generated by a pump driven by an electric motor.
2. Description of Related Art
Power steering apparatuses are conventionally used which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. In such a power steering apparatus, the oil pump is driven by an electric motor such as a brushless motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor.
Drive control of the electric motor is performed, for example, on the basis of a steering angular speed which is a time-related change amount of a steering angle observed when a steering wheel is operated. As shown in FIG. 6, if a steering angular speed W.theta. is not greater than a first threshold W1, a motor rotation speed R is kept at a first rotation speed R1. If the steering angular speed W.theta. is not smaller than a second threshold W2 which is greater than the first threshold W1, the motor rotation speed R is kept at a second rotation speed R2 which is greater than the first rotation speed R1. If the steering angular speed V.theta. is within a range between the first threshold W1 and the second threshold W2, the motor rotation speed R is varied generally linearly with the steering angular speed W.theta. between the first rotation speed R1 and the second rotation speed R2.
Thus, a greater steering assist force is generated as the steering wheel is operated more quickly. Hence, the steering assist can properly be provided.
When the steering wheel is returned to a steering angle midpoint (a steering angle which corresponds to an angular position of the steering wheel in straight traveling) after having been turned in one direction, a self-alignment torque is exerted on the steering mechanism due to a reverse input from tires to return the steering wheel to the steering angle midpoint. Therefore, a great steering assist force as required to turn the steering wheel is not necessary.
Nevertheless, the conventional power steering apparatus is adapted to perform the aforesaid control irrespective of the direction of the operation of the steering wheel. Even if a great steering assist force is not required, the rotation speed of the electric motor is increased in response to abrupt operation of the steering wheel. Therefore, wasteful energy consumption is inevitable.
Further, smooth return of the steering wheel to the steering angle midpoint by utilizing the self-alignment torque may be prevented in the conventional power steering apparatus.